The construction of a liquid crystal display device in conventional art is shown in FIG. 1. An ordinary liquid crystal display device is comprised of a backlight 11 of an edge light type on the furthest back surface and, in the order from the furthest back surface, a light introductive plate 12 for injecting light from the back light toward the surface, a scattering sheet 13 for uniformly dispersing brightness of the light, and one or plural light-tuning sheet (light tuning film) 14 having a function for condensing the uniformly dispersed light by the scattering sheet to a given direction or alternatively a function for selectively transmitting or reflecting a specific polarized light. Light passing through these films is injected to a liquid crystal cell 17 interposed between a pair of polarizing plates 15 (backside polarizing plate) and 16 (surface polarizing plate). The polarizing plate is comprised usually of three layers of a polarizing layer (polarizer) interposed by 2 sheets of a protecting film. In the figure, 18 denotes a cooled cathode fluorescent tube as light source and 19 a reflective sheet.
The light tuning film 14 and the backside polarizing plate 15 located on the side of the liquid crystal cell are especially not bonded with a binder or the like so that a slight gap exists between both. This light tuning film 14 is made of an acrylic resin, a polyester, a polycarbonate or the like, but these materials are rather larger in stretching or shrinking caused by change in temperature so that the light tuning film elongated by heating due to ambient circumstance, backlight or the like is brought into contact with the backside polarizing plate 15 to cause non-uniformity in display in circumferential areas of image. In some of the light tuning films, there exists a unique brightness non-uniformity, thus bringing about deterioration in their display quality.
JP-A (“JP-A” means unexamined published Japanese patent application) No. Hei. 10-240143 discloses that non-uniformity in display due to contact can be improved by imparting matt property forming concavo-convex pattern onto the surface. However, control of the concave-convex pattern was not as yet controlled so that a satisfactory improved effect has not been obtained. Moreover, the transmission rate of backlight is decreased in this method due to scattering of the concave-convex surface to incur lowering of brightness for display. As for non-uniformity in brightness of the light tuning film, the use of one more scattering film is thinkable between the light tuning film and the liquid crystal cell. As the scattering film generally has haze, the transmittance will be decreased to lower display brightness as in case of imparting matt property.
In case the matt property is imparted to a film, it is general that a hard coat layer is incorporated with particles of matt property to exhibit the matt property. This hard coat layer is also provided with a function to improve scratch-resisting property so that the hard coat layer is generally made of a rigid material such as a crosslinked binder polymer. Usually, a binder polymer is allowed to crosslink after it has formed a hard coat layer. For this, however, the binder polymer may be shrunk in the course of crosslinking reaction to permit the formation of crack in the hard coat layer. Further, as the hard coat layer shrinks as a whole, a film per se provided with the hard coat layer may undergo deformation (the generation of curl, etc.). In case such film is used as the aforesaid light tuning film, defect or strain is formed to deteriorate display quality.
In view of the foregoing, a liquid crystal display device employing a conventional optical film of matt property failed to be satisfactory in display brightness and display quality.
The display type of LCD can roughly be classified into a birefringence mode and an optical rotation mode. A super twisted nematic liquid crystal display device utilizing the birefringence mode (referred to hereinafter as STN-LCD) employs super twisted nematic liquid crystal possessing a twisted angle exceeding 90° and steep electrooptical characteristics. Therefore, STN-LCD enables display of a large capacity due to multiplex drive. However, STN-LCD has problems such as a slow response (several hundred milliseconds) and difficulty in grade display, and is inferior as compared with a liquid crystal display characteristics using active device (such as TFT-LCD and MIM-LCD).
In TFT-LCD and MIM-LCD, a twisted nematic liquid crystal possessing a twisted angle of 90° and a positive birefringence is used for displaying images. These are a display mode of TN-LCD which is an optical rotation mode. As this mode obtains a high responsibility (several ten milliseconds) and a high contrast, this mode is advantageous in many aspects as compared with the birefringence mode. Since TN-LCD changes display colors and display contrast according to a viewing angle of looking at the liquid crystal display device (viewing angle characteristics), it involves a problem that the device is difficult in watching as compared with CRT.
JP-A Nos. Hei. 4-229828 and Hei. 4-258923 disclose a proposal of providing a phase differential plate (optical compensative sheet) between a liquid crystal cell and a pair of polarizing plate for improving viewing angle characteristics. As the phase differential plate proposed in the aforesaid publications is a phase difference is almost 0 in the vertical direction to the liquid crystal cell, it gives no optical effect on direct front but a phase difference is realized when is tilted. A phase difference generated in an inclined direction is thereby compensated. A sheet having a negative birefringence so as to compensate a positive birefringence of a nematic liquid crystal and having an inclined optic axis is effective for such optical compensative sheet.
JP-A No. Hei. 6-75115 and EP 576304A1 disclose an optical compensative sheet having a negative birefringence and an inclined optic axis. This sheet is manufactured by stretching a polymer such as polycarbonate or polyester and has a main refractive index direction inclined to the normal line thereof. As such sheet requires an extremely complicate stretching treatment, however, it is extremely difficult to manufacture a uniform optical compensative sheet of a large area stably according to this method.
On the other hand, JP-A Nos. Hei. 3-9326 and 3-291601 disclose a method using a liquid crystalline polymer. An optical compensative sheet is thereby obtained by applying a liquid crystalline polymer onto the surface of an alignment (oriented) layer of a support. As the liquid crystalline polymer fails to show a satisfactory direction on the alignment layer, however, it is impossible to enlarge the viewing angle in all directions. JP-A No. 5-215921 discloses an optical compensative sheet (birefringent plate) comprises a support and a liquid crystalline polymeric bar-type compound having a positive birefringence. This optical compensative sheet is obtained by applying a solution of the polymeric bar-type compound onto the support and curing the compound under heating. However, the liquid crystalline polymer is devoid of birefringence so that it is unable to enlarge the viewing angle in all directions.
In JP-A No. Hei. 8-50206, there is disclosed an optical compensative sheet characterized by a layer of a negative birefringence comprised of a compound having a discotic structure unit wherein an angle between the discotic compound and a support is changed in the direction of the depth of the layer. According to the method described therein, a viewing angle viewed from contrast is extensively enlarged in all directions and deterioration of images such as yellowing viewed from an incline direction is scarcely observed. With the optical compensative sheet alone, however, a Newton ring caused by contact with the light tuning film and a non-uniformity in brightness caused by the light tuning film cannot be improved. Thus, further improvement is required.